|I know this is a post that is few months old, but the asker's question was never fully answered.|
From www.webdevelopsnotes.com :
You should keep in mind that the animated jpg is not one image but a series of images that are displayed in the same place in the browser.
We start by preloading all the jpg images required in the "animation".
The second step is to use a function that changes the source of an image. Finally, we call this function repeatedly by employing the setTimeout() method.
var c = 1;
/* Preloading images */
var image1 = new Image();
image1.src = "a1.jpg";
var image2 = new Image();
image2.src = "a2.jpg";
var image3 = new Image();
image3.src = "a3.jpg";
var image4 = new Image();
image4.src = "a4.jpg";
var image5 = new Image();
image5.src = "a5.jpg";
if (c == 6)
c = 1;
var img_src = "a" + c + ".jpg";
document.ani.src = img_src;
t = setInterval("disp_img(c)", 1000);
The entire script is placed inside the HTML document HEAD section.
We start by initializing a global variable c that serves as a counter and is also a part of the image name. After preloading the images (an1.jpg, an2.jpg, an3.jpg, an4.jpg and an5.jpg), we define a function disp_img() that changes the source of images named ani. The function also checks the value of variable c, assigning it a value of 1 if its value exceeds 5. The variable is also incremented each time the function is called. The function is called after every 1 second (1000 milliseconds) with the setInterval() method.
You can customize the script and create .jpg slide-shows easily.